Method of determining the optical axes of quartz crystals



y 9 R. J. WATROQISKI 2,423,357

METHOD OF DETERMINING THE OPTICAL AXIS OF A QUARTZ CRYSTAL Filed Jan.13, 1942 Inventor-1 Raymond J"..Wa.z1-obaki,

tal trigonally symmetrical.

Patented July 1, 1947 METHOD OF DETERMINING THE OPTICAL AXES OFQUARTZCRYSTALS Raymond J. Watrobski, Schenectady, N. Y., as

signor to General Electric Company, a corporation of New YorkApplication January 13, 1942, Serial No. 426,640

: 4 Claims. 1

My invention relates to crystals, and more particularly to a method fordetermining the optical axis of crystals.

Quartz crystals, as they occur in nature, have a. typically crystallineform, which may be described as a hexagonal prism terminated at each endby structures which are roughly in the form of hexagonal pyramids. Theroughly pyramidal end structures of the hexagonal prism have three majorand three minor faces, and make thecrys- The axis of trigonal symmetryis approximately parallel to the fixed edges of the hexagonal prism, andis called the optical axis of the quartz crystal, because quartzpossesses the property of rotating the plane of polarization of planepolarized light traversing the crystal parallel to this axis. Y 7 gQuartz crystalspossess two other sets of characteristic axes, both ofwhich are perpendicular to the optical axis. One set of such axes iscalled mechanical, while the other set is called electric; Eachmechanical axis is approximately perpendicular to two opposite faces ofthe hexagonal prism. Each electric axis is perpendicular to the opticalaxis and to a mechanical axis.

The electrical and other characteristics of pieces. of quartz cut fromsuch quartz crystals depend primarily on the orientation of the cuttingplanes with respect to the optical, electric and mechanical'axes of thecrystal; In order, therefore, to cut quartz crystals into satisfactorypieces, it is necessary to know accurately at least two of thecharacteristic axes, the third being readily determinable thereafter.

It is accordingly an object of my-invention to provide a newand improvedmethod for determining more readily the optical axis of a'crystal.

Most high quality quartz crystals, particularly those suitable for usein making piezoelectric vibrators, are of irregular shape as purchasedin commercial channels. A small portion of such crystals is riverquartz, whose outer surfaces have been worn away by tumbling in a; riverbed, so

rotate about its vertical axis. Mounted on the upper end of the shaft IDon a swivel joint allow- 2 viously more difficult to determine suchoptical axis for river quartz or mutilated quartz as described above. Itis accordingly a further object of my invention to provide a new andimproved method for determining readily the optical axis of acrystaleven though its characteristic outer surface has been mutilated or wornaway.

It is also an object of my invention to provide such a method which doesnot depend upon the use of polarized'light, and which is capable'ofgreater accuracy in the determination of the optical axis than'methodsutilizing polarized light.

It is still another object of my invention to provide such a methodwhich is particularly useful for determining the optical axis of aquartz crystal. The features of my invention which I believe to be novelare set forth with particularity in the appendedclaims, -My inventionitself, both as to its organization and manner of operation, togetherwith further objects and advantages thereof may best be understood byreference to the following description taken in connection with theaccompanying drawing in which the single figure is a perspective view,partly in section, of apparatus suitable for use in connection with myinvention.

.In-the figure a hollow cylindrical shaft III is supported verticallythrough a workbench, or table, ll by a bearing, generally designated",which is so arranged as to allow the shaft H) to ing limited rotationalmotion in any direction is a support I3 having three arms l4, l5 and [6extending'outwardly from a point above the upper end of the shaft H). Aslidable bracket I! carryshown, are so arranged as to be capable ofmaintaining an irregularly shaped body, such as the pieceZl of quartzcrystal, mounted in a fixed position with respect to the support 13. Thesupport [3 is so arranged with respect to the vertical shaft III that itnot only rotates with the shaft ill but also allows adjustment of theangles between the arms I 4 and I5 and the axis of the shaft Ill. Thearms l4 and [5 lie in angularly displaced planes passing through theshaft Ill and are an-v gulariy adjustable with respect to the shaft sothat the irregularly shaped body 2i may be oriented in any desiredmanner with respect to the axis of the shaft I0.

The adjustment of the angle between the arm l4 and the axis of the shaftI is produced by adjustment of the control knob 22, whether the shaftIII, the support l3 and the crystal 2| are rotating or not. .The linkageconnecting the control knob 22 and the arm |4 includes a lever 23mounted on a pivot 24 near the surface of the hollow cylindrical shaftIII for rotation about an axis perpendicular to the axis of the shaftIll. The outer end of the lever 23 is maintained at a fixed distancefrom the outer end of the arm H by a link 25, pivotally connectedrespectively to the link 23 and the arm H. The end of the lever 23within the shaft II! is fork-shaped, and is pivotally connected with abushing 23 connected through a bearing 21 to the upper end of a hollowshaft 28. The bearing 21 allows rotational movement between the shaft 28and the bushing 23, but prevents any axial movement of the bushing 28along the shaft 28.

The hollow shaft 28 is concentric with the hollow shaft l0, and ismounted in bearings 29 and 30 within the shaft l0 toallow the shaft 28to be moved axially with respect to the shaft l0, or to allow rotationalmovement therebetween. The lower end of the shaft 28 is pivotallyconnected to a. lever 3| which is pivotally supported from the table Ion a support 32. The opposite end of the lever 3| is directly beneaththe control knob 22. and is pivotally connected to a screw-threadedblock 33 carried by a threaded shaft 34 attached to the control knob 22.

Rotation of the control knob 22 in a clockwise direction moves the block33 upward and, through the medium of the lever 3| moves the hollow shaft28 downward. Such movement of the shaft 28 moves the forked end of thelever 23 downward through the medium of the bearing 21, reardless ofrotation of the shaft l0 and lever 23 around the axis of the shaft Hi.When the forked end of the lever 23 moves down, its outer end moves up,and through the link 25, moves the outer end of the arm l4 upward.

For accurate work all bearings and connections in the linkage describedshould be carefully made and adjusted to have a minimum amount of play.

Thisis necessary in order that the orientation of the irregularly shapedbody 2| may be adjusted within smalllimits, and so that such adjustmentshall be maintained regardless of rotation of the A similar ilnkage isprovided to move the outer end of the arm |5 upward when a secondcontrol knob 41 is turned in a clockwise direction. This linkageincludes a second lever 35 pivotally mounted near the surface of thehollow shaft ill for rotation about an axis perpendicular to the axis ofthe shaft Ill. The outer end of the lever 35 is maintained at a constantdistance from the outer end of the arm |5 by means of a link 36,pivotally connected at its ends to the lever 35 and the arm l5. 3

The inner end of the lever 35, is forked, and is pivotally connected toa bushing 31 which'is constrained against axial movement with respect toa shaft 38 by means of a bearing 39. The bearing 33 is so arranged as toallow the bushing 31 and the shaft 38 to rotate with respect to oneanother.

The shaft 38 is arranged so that it may rotate within the shaft III bymeans of a bearing 40 in the shaft l0 and a second bearing comprisingthe inner surface of the hollow shaft 28. The shaft 33 extends downwardthrough the hollow shaft It is convenient to provide a motor drive formaintaining constant rotation of the body 2|. Such a drive may beprovided by means of a pulley 43 formed on the lowerend of the shaft illbeneath the table M. This pulley 43 is rotated by means of a belt 44driven from a pulley 45 on a driving shaft 46. The shaft I0 mayconveniently be arranged to rotate at relatively slow speed, forexample, a speed in the order of 10 to 100 R. P. M.

The arrangement described is particularly useful for rotating a quartzcrystal and for orienting it about its axis of rotation during rotationthereof until its optical axis coincides with its axis of rotation. Inorder to determine when its optical axis coincides with its axis ofrotation, light is utilized, and I provide a light source 50 and aground glass light receiving screen 5|, to be used in a manner to beexplained later. The light source 50 may conveniently be any smallincandescent lamp energized from a suitable source and mounted in aholder with a collimating lens, in sucha way as to project a beam 52 oflight upon an upper surface 53 of the quartz crystal 2| at a suitableangle. Ideally, the light beam 52 is coincident with the axis ofrotation of the shaft ||I so that the reflected beam retraces the samepath, or is displaced at only a slight angle in the event that it isfound necessary to adjust the crystal so that the surface 53 is slightlyoblique to the shaft axis. Such an arrangement, while possible, isdifficult of attainment, and in" the preferred-embodiment of theinvention shown in the drawing, the direct beam 52 and the reflectedbeam are each at an angle of about 15 with the axis of the shaft Ill, sothat the physical structure of the light source 50 does not interceptthe reflected beam.

In order to hold the source 50 of light in a suit able position, and atsuch an angle, a bracket I 49, adjustably mounted on a post 54 fixed tothe table II may be provided.

It is preferred that the ground glass light receiving screen 5| bepositioned directly above the upper surface 53 of the quartz crystal 2|,and it may be so positioned by being mounted on a bracket 55 fastened toa post 56 fixed on the table II. The brackets 49 and 55 are fastenedrespectively to the posts 54 and 56 by adjustable means so that they maybe, placed respectively at any desired height above the crystal 2 lightis not necessary, but does usually make it unnecessary to cut more thanone face 53 on the quartz crystal 2|.

After the face 53 is cut or ground on the crystal 2 I, if the crystal isquartz, it is etched in 48% hydrofluoric acid for about eight hours. Itis believed that this etching is most effective in dissolving thestructure of a quartz crystal where Y near the surface 53 are removed,leaving only one side of the e Any crystal so etched refle there is noplane of crystal cleavage, witlr the therewith, each tail facing outwardfrom the result that irregularly shaped pieces of crystal center of thepattern and being anextension of quilateral triangle formed by smallregularly shaped crystalline elements. the faint dots. The sides of-bothequilateral tri- These regularly shaped crystal elements are all angles,and the comet-like tails of the faint dots, oriented in the samedirection, and bear a definite lie approximately in the direction of theelectric relation to the orientation ofthe optical axis of axes of thecrystal. The comet-like tails of, the the crystal as a whole. Thesesmallcrystal elefaint dots lie toward the negative direction along mentsare generally of pyramidal shape and rethe electrical axes. The facesofthe crystal are fleet light in a unique pattern. approximatelyparallel to the sides of the two If the crystal 2| isof material otherthan equilateral triangles. quartz, the etching'should be done bysomeo'ther The three primary dots are usually about A suitable solventfor the particular crystal ma! inchin diameter when the frosted glass isabout terial. Such suitablesol ent shouldbeeflective 6 inches from theupper surface '53 of the crysto etch the surface of theparticularcrystal and. 5 tel 2|. The pattern as a whole in such casehasleave its elemental crystal structure exposed. a diameter of'about oneinch. Ii thecrystal 2| ctslight ina charbe kept stationary and the beam52 of-iight be moved over the surface 53, twinningis revealed,

After the face 53 of the crystal 2| is so etched, when the beam 52 oflight passes between dextrothe crystal is mounted as explainedpreviously on go rotatory and levorotatory portions of the crysthesupport l3, with the face 53 substantially tal, by a sudden reversalofthe light pattern on horizontal. When the source 50 of light is. enthefrosted screen 5|. In this reversal the three ergized, to project thebeam 52, of light on the primary dots 60, Gland 62 take the place of thesurface 53 of the crystal 2|, such light is reflected three faint dotswith the comet-like tails, and the from the surface 53 and, if thecrystal is quartz, t ree faint dots take the place of t e three Dacteristic pattern.

appears on the ground-glass 5| as three spots of mary dots.

light 60, 6| and 52. These three spots of light Twinnine" exists when acrystal has two pertions, one of which is dextrorotatory, or capable maybe regarded as situated at the apices of a triangle. If the beam oflight 52 impinging on the O rotating e Plane f polarization of light insurface 53 were pendicular thereto, the three spots one direction. andthe other portion o which is of light 60, 6| and '62 would be situatedatth levorotatory, or capable of rotating the plane of in the otherdirection.

apices of an equilateral triangle. When the beam polarization of light vAs set forth above, crystals other than quartz,

52 impinges at some large angle, near 90 to the surface 53, the threespots 60, 6| and 62 of light w Suitably e r f l ght o form harform atriangle of which one side is slightly fi patterns- The Opticalaxis ofsuch crysshortened, but the triangle is still essentially an tals y bedetermined in a'manner Similar to equilateral triangle. It has beenfound that an t Of quartz crystals, by Projecting a beam angle of 15,suggested above, between t b a of light on the etched face so that areflected 52 of i t and a perpendicular t t surf c 53 light pattern isformed, and determining the axis produces spots 60, 6| and 62 of lightwhich form 40 about which the crystal may be rotated to cause anapproximately equilateral triangle. such reflected light pattern torotate about its Now, t determine t optical axis, t crystal center. Whenthe reflected light pattern rotates 2 is rotated by h means aspreviously about its center, the axis of rotation coincides scribed, orotherwise, about the vertical axis with e Optical axis-0f the crystal:When I use perpendicular to, the frosted glass 5|, and the the termcrystal herein, I do not intend to include metallic conducting bodies.Such bodies light pattern, such as the spots 60. 6| and 62pfor' int. Solong as the 2 cannot have anoptical axis.

quartz, rotates about a po 7 optical axis of the crystal 2| does notcoincide While I have shown anddescribed a particular with its axis ofrotation, the point about which em m nt f my i v n it will be obviou thepattern rotates does not coincide with the those skilled n the art thath e and center of the pattern. The control knobs 22 and modificationsmay be made without departing in its broader aspects.

61 are then adjusted, thereby changing the from'my invention orientationof the crystal about its rotational What I claim asnew and desire tosecure by axis, until the optical axis coincides with such LettersPatent of the United States is:

rotational axis, at which time, if the crystal is 1. The method ofdetermining the optical axis quartz, thespots 60, 6| and 62 rotate aboutthe of a quartz crystal which comprises determining center of thetriangle formed thereby, such optical axis roughly by means of polarizedWhile it has been specifled that 48% hydrolight, forming a plane surfaceon said crystal perfluoric acid may be used to etch the face 53 of apendicular to such roughly determined optical quartz crystal 2|, eitherweaker or stronger soluaxis, etching said plane surface, projecting ations of the acid may be utilized, the time of beam of light on saidsurface at a small angle etching being correspondingly increased ordefrom the perpendicular to said surface for refleccreased. If theetching f q rtz is shallow. only tion from said surface to produce alight patthe three dots 60, 6| and 62 of light, as described tern,rotating said crystal about an axis subabove, may be seen. If theetching is carried out 65 ,stantially perpendicular to said surface, andas described previously, with 48% hydrofluoric changing the orientationof said crystal with reacid for about eight hours, generally threeadspect to the rotational axis until the reflected ditional faint dotsof light may be seen forming light pattern rotates about its own center,the an equilateral triangle reversed 180 from that rotational axis ofthe crystal then coinciding formed by the dots 60, 6| and 62. The amount7 with the optical axis. of etching required before these threeadditional 2. The method of accurately determining the faint dots may beseen depends on. the particoptical axis of a crystalline element whichcomular structure of the crystal being examined. prises approximatelydetermining said optical These three faint dots of light, when seen axisby means of polarized light, forming upon clearly, have comet-like tailsof light associated said element a plane surface substantially per-- 7pendicuiar to said approximately determined optical axis, etching saidplane surface, projecting a light beam upon said etched surface,receivmg upon a second plane surface in substantially parallel spacedrelation to said etched surface a light patterndetermined by 9,characteristic of said etched surface, rotating said crystalline elementabout said approximately determined optical axis and observing themovement of said pattern upon said second surface, and changing the axisof rotation of said element to eifect rotation of said pattern about itsown center thereby to render the axis of rotation of said elementcoincident with its optical axis.

3. The method of accurately determining the optical axis of apiezoelectric crystal which comprises approximately determining saidoptical axis by means of polarized light, forming upon said crystal aplane surface substantially perpendicular to said approximatelydetermined optical axis, etching said plane surface, projecting a lightbeam upon said crystal so that a beam projected substantiallyperpendicularly from said etched surface forms upon a second surface insubstantially parallel spaced relation with said etched surface a lightpattern comprising three distinct dots juxtaposed in accordance with acrystalline characteristic of said etched surface, rotating said crystalabout said approximately determined onticalaxis to effect rotation ofsaid pattern upon said second surface, and changing the rotational axisof said crystal to effect rotation of said pattern about its own center,the optical axis of said crystal thereupon coinciding with itsrotational axis,

prises approximatelis determining said optical axis by means ofpolarized light, forming upon said element a plane surface substantiallyperpendicular to said approximately determined optical axis, etchingsaid plane surface, reflecting a light beam substantiallyperpendicularly from said etched surface to form upon a second surfacein substantially parallel spaced relation to said etched surface alightpattern determined in accordance with a characteristic of said etchedsurface, rotating said crystalline element about said approximatelydetermined optical axis to effect rotation of said pattern upon saidsecond surface, and changing the axis of rotation of said crystallineelement to effect rotation of said pattern about its own center, theoptical axis of said element thereupon coinciding with its rotationalaxis; I

RAYMOND J. WATROBSKI.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS method of accurately determining the

